The reason the update is so massive is that it's not limited to just heists. Rockstar announced earlier this week that the patch includes new competitive modes and features for GTA 5 as well.

The players that figured out how to access the patch early can't actually perform these cooperative heists yet. The actual content of the patch is currently inaccessible. I do hope that Rockstar gives everyone the opportunity to preload these files, though, so everyone doesn't spend the first days of heists simply waiting for the download to complete.